feat(ui): show progress bar animation while models are loading

[Pfannkuchensack]

Summary

Model load events are already emitted by the backend but were only logged in the frontend. Track a loading counter via model_load_started / model_load_complete and flip the existing ProgressBar into indeterminate mode while any model is loading, so users get visual feedback that something is happening.

Related Issues / Discussions

https://discord.com/channels/1020123559063990373/1493251617636749443

QA Instructions

1. Start the app with a fresh backend (so the model cache is empty).
2. Trigger a generation that requires loading a model from disk (e.g. switch to a model that isn't currently in the RAM cache).
3. Observe the progress bar under the Invoke button:
   • Before load: idle (value 0, not animated).
   • While the backend is loading the model(s): indeterminate animation.
   • Once generation starts: normal step-progress behavior as before.
4. Verify behavior with multiple submodels (e.g. SDXL / FLUX) — the counter handles parallel loads.
5. Disconnect/reconnect the socket mid-load: the counter resets to 0, no stuck animation.

Merge Plan

Standard merge. No DB / schema / redux slice changes.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable) — no unit-testable logic, UI-only change
• ❗Changes to a redux slice have a corresponding migration — N/A (nanostore atom, not redux)
• Documentation added / updated (if applicable) — N/A
• Updated What's New copy (if doing a release after this PR)